In numerous industrial applications, it is desirable to determine wear and tear of metallic parts. The metallic parts could be an inside bore of an artillery cannon, metallic parts of an aircraft engine or its body, metallic parts exposed to thermal cycling (periodic exposure to extreme heat and low temperatures, as in outer space) or a body of a car or any machine, for example.
Due to wear and tear, regions in material develop structural damage that may eventually develop into physical defects like cracks (either surface or sub-surface cracks or cracks deep inside the material). Such cracks can compromise mechanical stability of a structure as well as possibly degrade performance. Some regions with structural damage may not be directly optically visible because physical cracks may not have developed yet. Such notionally ‘invisible’ damaged regions are locations where material density, composition, or other material physical properties have become modified slightly compared to regions without damage (for example, in regions where corrosion may have just begun or nucleated, or regions that are subjected to stresses and loads which may lead to cracks or mechanical failure of the material).